This invention relates to borehole logging devices of the type wherein electromagnetic energy is used for measuring properties of formations surrounding a borehole and, more particularly, to an improved antenna shield for use in such devices to reduce the effect of spurious modes of the electromagnetic energy.
Induction logging has been employed for many years for measuring the conductivity of subsurface earth formations surrounding an earth borehole. In conventional induction logging a number of coils are wound on a mandrel. One or more transmitter coils are energized by an alternating current at a frequency such as 20 KHz. The resultant oscillating magnetic field causes induction of circulating currents in the formations which are substantially proportional to its conductivity. These currents, in turn, cause a voltage to be induced in receiver coils, and the conductivity of the formations is determined from the induced voltage.
Shields have been employed in the prior art in conjunction with induction logging devices for purposes such as the reduction of the effects of electrostatic coupling between the formations (or borehole fluid) and the coil being shielded.
For example, in U.S. Pat. No. 2,623,923 there is disclosed a shield that has a number of conductor wires parallel to the axis of the borehole and enclosing the antenna coil, the wires being open at one end and terminating at the other end in a common split-ring conductor. This construction prevents completed current paths in the shield. Another type of known induction logging shield employs a structure similar to that just described, but with the wires being replaced by thin strips on a "printed circuit" type of sheet that is wrapped around the induction logging antenna. These types of shields can be useful in a number of induction logging applications. However, existing shielding techniques are inadequate for the special problems that are encountered in the type of logging device to be described.
In recent years logging systems have been proposed for employing radio frequency electromagnetic energy in the range between about 10 MHz and 100 MHz to determine both the dielectric constant and the conductivity of formations surrounding a borehole. In this frequency range, dielectric constant and conductivity both have a substantial effect upon the propagation constant of electromagnetic energy propagating in the formations, so measurements of attenuation and phase can be used for solution of simultaneous equations to determine the dielectric constant and/or conductivity of formations through which the electromagnetic energy has passed. A device of this type is the deep propagation tool ("DPT"), an embodiment of which is described in U.S. Pat. No. 4,209,747. This device includes a transmitting antenna, a "close" receiver antenna pair, and a "far" receiver antenna pair. Each of the transmitter and receiver antennas are coils wound in insulating media mounted on a metal cylindrical pipe which carries wiring to and/or from the coils. Briefly, operation of the DPT logging device involves energizing the transmitter to emit electromagnetic energy at a frequency suitable for determination of both the electrical conductivity and the electrical permittivity of the surrounding formations. A portion of the electromagnetic energy which has traveled through the formations is received at the close and far differential receiver pairs. The signals detected at the far receiver pair are used to determine the phase shift of electromagnetic energy that has passed through the formations, and the signals detected at the close receiver pair are used to determine relative attenuation of the electromagnetic energy. The phase shift and attenuation are then employed to obtain electrical permittivity and electrical conductivity of the formations.
The relatively high frequency electromagnetic energy that is used to obtain the substantial displacement currents needed to measure dielectric properties of the formations attenuates quickly as it travels through the formations. The receivers are typically spaced a substantial distance from the transmitter to attain a significant depth of investigation into the formations. Accordingly, the signal levels received at the receivers (particularly the far receiver pair) tend to be weak, especially in relatively conductive (lossy) formations. Since it is necessary to obtain accurate measurements of the relative attenuation and phase at the receivers, it is desirable to have the signal-to-noise ratio at said receivers be as high as possible. When the received signal is weak, however, as is often the case, the amount of spurious or interfering signal (i.e., "noise") is a limiting factor on the measurement accuracy of the logging device.
There are various phenomena which cause noise at the receivers, but applicant has discovered that there is an important interfering signal in a DPT type of logging device resulting from electromagnetic energy that propagates in the region between the central metal pipe of the device and the borehole fluid of the formations. The energy that propagates in this region has a predominant transverse magnetic component that can be envisioned as propagating in a coaxial type of transmission line. To picture this mode of propagation, the metal cylindrical pipe of the logging device can be thought of as the central conductor of a coaxial line, and the borehole fluid can be thought of as the outer cylindrical conductor of the coax. The region in which the wave propagation takes place is roughly analagous to the air-filled or dielectric-filled region of a coaxial transmission line. [If the borehole fluid is not very conductive (for example, a fresh water-based mud or an oil-based mud), the formations may be thought of as constituting the outer conductor in the coaxial transmission line model.] If the central pipe and the drilling fluid were perfect conductors, the electromagnetic energy propagating in the region therebetween would be a transverse electromagnetic ("TEM") mode wave having a radial electric field and an azimuthal magnetic field. Depending upon the relative conductivity of the outer "conductor" in the coaxial transmission line model (i.e., the borehole fluid, the mudcake, or the formations, as the case may be), the propagating noise mode may comprise another type of electromagnetic wave also having a predominant transverse magnetic component; i.e., a transverse magnetic ("TM") mode electromagnetic wave that includes both radial and axial electric field components, and an azimuthal magnetic field component. The propagating noise modes having a predominant transverse magnetic component (i.e., both the TEM and the TM type modes) will be referred to herein as transverse magnetic modes or components.
If the transmitter and receiver coils were perfectly configured and balanced in a theoretically ideal system, the electromagnetic wave energy generated by the coils would be transverse electric ("TE") mode, of the type generated by an ideal vertical magnetic dipole. However, under actual operating circumstances there is sufficient misalignment, unbalance, or other conditions that give rise to significant undesired transverse magnetic mode. As described hereinabove, the logging device itself in the borehole can act as a coaxial transmission line for transverse magnetic wave energy which can thereby propagate relatively unattenuated between the transmitter and the receivers.
It is an object of the present invention to improve operation in the described type of logging devices, and in induction logging devices in general, by substantially reducing the transmission and/or reception of spurious signals such as transverse magnetic mode noise.